typedef void (* XPLMHandleKey_f)(
                         XPLMWindowID         inWindowID,    
                         char                 inKey,    
                         XPLMKeyFlags         inFlags,    
                         char                 inVirtualKey,    
                         void *               inRefcon,    
                         int                  losingFocus);    

This function is called when a key is pressed or keyboard focus is taken away from your window. If losingFocus is 1, you are losing the keyboard focus, otherwise a key was pressed and inKey contains its character. You are also passed your window and a refcon.

Warning: this API declares virtual keys as a signed character; however the VKEY #define macros in XPLMDefs.h define the vkeys using unsigned values (that is 0x80 instead of -0x80). So you may need to cast the incoming vkey to an unsigned char to get correct comparisons in C.